In this connection, it is known from U.S. Pat. No. 5,197,326 to evaluate the on-board voltage for diagnosing an rpm transducer. The drop and subsequent increase of the on-board voltage is detected by the control apparatus, that is, the battery voltage when actuating the starter. When the control apparatus detects such a voltage trace, the conclusion is drawn that a start operation has taken place. The output signal of the rpm transducer, which is to be monitored, then has to permit recognition that the crankshaft rotates. If the output signal of the rpm transducer does not change, then a defect of the transducer or a line interruption must be present.
DE 44 22 149 discloses how, for example, the operability of a lamp during normal operation of the vehicle can be checked by evaluating the on-board voltage. Disturbances are superposed on the on-board voltage when switching on and switching off individual consumers in normal operation of the motor vehicle which disturbances are caused by the actuation of other consumers. As an example, the injection and ignition signals are mentioned which regularly occur in a running engine. To ease this situation, DE 44 22 149 provides a modeling of the on-board voltage in the operation and/or a mathematical convolution of the measured on-board voltage with a jump-shaped signal. From this, an independence is intended to be achieved from direct-current voltage components and a reduced sensitivity against high-frequency disturbances such as occur in the operation of the vehicle.